content: /* Copyright (C) 2016-2018 Alibaba Group Holding Limited
  
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at
  
      http://www.apache.org/licenses/LICENSE-2.0
  
  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.
  ==============================================================================*/
  
  // Protocol Buffers - Google's data interchange format
  // Copyright 2008 Google Inc.  All rights reserved.
  // https://developers.google.com/protocol-buffers/
  //
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are
  // met:
  //
  //     * Redistributions of source code must retain the above copyright
  // notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
  // copyright notice, this list of conditions and the following disclaimer
  // in the documentation and/or other materials provided with the
  // distribution.
  //     * Neither the name of Google Inc. nor the names of its
  // contributors may be used to endorse or promote products derived from
  // this software without specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  package com.google.protobuf;
  
  import static java.lang.Math.max;
  import static java.lang.Math.min;
  
  import java.io.IOException;
  import java.io.OutputStream;
  import java.lang.ref.SoftReference;
  import java.lang.reflect.Field;
  import java.nio.ByteBuffer;
  import java.nio.channels.WritableByteChannel;
  
  /**
   * Utility class to provide efficient writing of {@link ByteBuffer}s to {@link OutputStream}s.
   */
  final class ByteBufferWriter {
    private ByteBufferWriter() {}
  
    /**
     * Minimum size for a cached buffer. This prevents us from allocating buffers that are too
     * small to be easily reused.
     */
    // TODO(nathanmittler): tune this property or allow configuration?
    private static final int MIN_CACHED_BUFFER_SIZE = 1024;
  
    /**
     * Maximum size for a cached buffer. If a larger buffer is required, it will be allocated
     * but not cached.
     */
    // TODO(nathanmittler): tune this property or allow configuration?
    private static final int MAX_CACHED_BUFFER_SIZE = 16 * 1024;
  
    /**
     * The fraction of the requested buffer size under which the buffer will be reallocated.
     */
    // TODO(nathanmittler): tune this property or allow configuration?
    private static final float BUFFER_REALLOCATION_THRESHOLD = 0.5f;
  
    /**
     * Keeping a soft reference to a thread-local buffer. This buffer is used for writing a
     * {@link ByteBuffer} to an {@link OutputStream} when no zero-copy alternative was available.
     * Using a "soft" reference since VMs may keep this reference around longer than "weak"
     * (e.g. HotSpot will maintain soft references until memory pressure warrants collection).
     */
    private static final ThreadLocal<SoftReference<byte[]>> BUFFER =
        new ThreadLocal<SoftReference<byte[]>>();
  
    /**
     * This is a hack for GAE, where {@code FileOutputStream} is unavailable.
     */
    private static final Class<?> FILE_OUTPUT_STREAM_CLASS = safeGetClass("java.io.FileOutputStream");
    private static final long CHANNEL_FIELD_OFFSET = getChannelFieldOffset(FILE_OUTPUT_STREAM_CLASS);
  
    /**
     * For testing purposes only. Clears the cached buffer to force a new allocation on the next
     * invocation.
     */
    static void clearCachedBuffer() {
      BUFFER.set(null);
    }
  
    /**
     * Writes the remaining content of the buffer to the given stream. The buffer {@code position}
     * will remain unchanged by this method.
     */
    static void write(ByteBuffer buffer, OutputStream output) throws IOException {
      final int initialPos = buffer.position();
      try {
        if (buffer.hasArray()) {
          // Optimized write for array-backed buffers.
          // Note that we're taking the risk that a malicious OutputStream could modify the array.
          output.write(buffer.array(), buffer.arrayOffset() + buffer.position(), buffer.remaining());
        } else if (!writeToChannel(buffer, output)){
          // Read all of the data from the buffer to an array.
          // TODO(nathanmittler): Consider performance improvements for other "known" stream types.
          final byte[] array = getOrCreateBuffer(buffer.remaining());
          while (buffer.hasRemaining()) {
            int length = min(buffer.remaining(), array.length);
            buffer.get(array, 0, length);
            output.write(array, 0, length);
          }
        }
      } finally {
        // Restore the initial position.
        buffer.position(initialPos);
      }
    }
  
    private static byte[] getOrCreateBuffer(int requestedSize) {
      requestedSize = max(requestedSize, MIN_CACHED_BUFFER_SIZE);
  
      byte[] buffer = getBuffer();
      // Only allocate if we need to.
      if (buffer == null || needToReallocate(requestedSize, buffer.length)) {
        buffer = new byte[requestedSize];
  
        // Only cache the buffer if it's not too big.
        if (requestedSize <= MAX_CACHED_BUFFER_SIZE) {
          setBuffer(buffer);
        }
      }
      return buffer;
    }
  
    private static boolean needToReallocate(int requestedSize, int bufferLength) {
      // First check against just the requested length to avoid the multiply.
      return bufferLength < requestedSize
          && bufferLength < requestedSize * BUFFER_REALLOCATION_THRESHOLD;
    }
  
    private static byte[] getBuffer() {
      SoftReference<byte[]> sr = BUFFER.get();
      return sr == null ? null : sr.get();
    }
  
    private static void setBuffer(byte[] value) {
      BUFFER.set(new SoftReference<byte[]>(value));
    }
  
    private static boolean writeToChannel(ByteBuffer buffer, OutputStream output) throws IOException {
      if (CHANNEL_FIELD_OFFSET >= 0 && FILE_OUTPUT_STREAM_CLASS.isInstance(output)) {
        // Use a channel to write out the ByteBuffer. This will automatically empty the buffer.
        WritableByteChannel channel = null;
        try {
          channel = (WritableByteChannel) UnsafeUtil.getObject(output, CHANNEL_FIELD_OFFSET);
        } catch (ClassCastException e) {
          // Absorb.
        }
        if (channel != null) {
          channel.write(buffer);
          return true;
        }
      }
      return false;
    }
  
    private static Class<?> safeGetClass(String className) {
      try {
        return Class.forName(className);
      } catch (ClassNotFoundException e) {
        return null;
      }
    }
    private static long getChannelFieldOffset(Class<?> clazz) {
      try {
        if (clazz != null && UnsafeUtil.hasUnsafeArrayOperations()) {
          Field field = clazz.getDeclaredField("channel");
          return UnsafeUtil.objectFieldOffset(field);
        }
      } catch (Throwable e) {
        // Absorb
      }
      return -1;
    }
  }
  